13.14.2 Open-time Flags

The open-time flags specify options affecting how open will behave.
These options are not preserved once the file is open. The exception to
this is O_NONBLOCK, which is also an I/O operating mode and so it
is saved. See Opening and Closing Files, for how to call
open.

There are two sorts of options specified by open-time flags.

File name translation flags affect how open looks up the
file name to locate the file, and whether the file can be created.

Open-time action flags specify extra operations that open will
perform on the file once it is open.

Here are the file name translation flags.

Macro: intO_CREAT

If set, the file will be created if it doesn’t already exist.

Macro: intO_EXCL

If both O_CREAT and O_EXCL are set, then open fails
if the specified file already exists. This is guaranteed to never
clobber an existing file.

Macro: intO_NONBLOCK

This prevents open from blocking for a “long time” to open the
file. This is only meaningful for some kinds of files, usually devices
such as serial ports; when it is not meaningful, it is harmless and
ignored. Often opening a port to a modem blocks until the modem reports
carrier detection; if O_NONBLOCK is specified, open will
return immediately without a carrier.

Note that the O_NONBLOCK flag is overloaded as both an I/O operating
mode and a file name translation flag. This means that specifying
O_NONBLOCK in open also sets nonblocking I/O mode;
see Operating Modes. To open the file without blocking but do normal
I/O that blocks, you must call open with O_NONBLOCK set and
then call fcntl to turn the bit off.

Macro: intO_NOCTTY

If the named file is a terminal device, don’t make it the controlling
terminal for the process. See Job Control, for information about
what it means to be the controlling terminal.

On GNU/Hurd systems and 4.4 BSD, opening a file never makes it the
controlling terminal and O_NOCTTY is zero. However, GNU/Linux systems
and some other systems use a nonzero value for O_NOCTTY and set the
controlling terminal when you open a file that is a terminal device; so
to be portable, use O_NOCTTY when it is important to avoid this.

The following three file name translation flags exist only on
GNU/Hurd systems.

Macro: intO_IGNORE_CTTY

Do not recognize the named file as the controlling terminal, even if it
refers to the process’s existing controlling terminal device. Operations
on the new file descriptor will never induce job control signals.
See Job Control.

Macro: intO_NOLINK

If the named file is a symbolic link, open the link itself instead of
the file it refers to. (fstat on the new file descriptor will
return the information returned by lstat on the link’s name.)

Macro: intO_NOTRANS

If the named file is specially translated, do not invoke the translator.
Open the bare file the translator itself sees.

The open-time action flags tell open to do additional operations
which are not really related to opening the file. The reason to do them
as part of open instead of in separate calls is that open
can do them atomically.

Macro: intO_TRUNC

Truncate the file to zero length. This option is only useful for
regular files, not special files such as directories or FIFOs. POSIX.1
requires that you open the file for writing to use O_TRUNC. In
BSD and GNU you must have permission to write the file to truncate it,
but you need not open for write access.

This is the only open-time action flag specified by POSIX.1. There is
no good reason for truncation to be done by open, instead of by
calling ftruncate afterwards. The O_TRUNC flag existed in
Unix before ftruncate was invented, and is retained for backward
compatibility.

The remaining operating modes are BSD extensions. They exist only
on some systems. On other systems, these macros are not defined.